1. Field of the Invention
The present invention relates to a piezoelectric element, more specifically, an improved piezoelectric element having good vibration characteristics in which unwanted signals based on unwanted vibrations (hereinafter, referred to as “spurious responses”) are suppressed effectively. The present invention also relates to an improved composite piezoelectric element in which unwanted vibration modes do not interfere with each other between adjacent piezoelectric elements. The present invention also relates to a filter, a duplexer and communication equipment using the same.
2. Description of the Background Art
There is a demand for increasingly smaller and lighter components to be integrated in electronic equipment, in particular portable equipment. For example, for filters used in portable equipment, there is a demand for compactness, small insertion loss and large attenuation characteristics.
Filters using a piezoelectric element are known as filters that can satisfy these requirements.
FIG. 27A is a schematic cross-sectional view of a conventional piezoelectric element. FIG. 27B is an equivalent circuit diagram of this conventional piezoelectric element.
As shown in FIG. 27A, the conventional piezoelectric element includes a piezoelectric vibrating portion 90 on a substrate 91. A piezoelectric layer 92, an upper electrode layer 93, and a lower electrode layer 94 are laminated to form the piezoelectric vibrating portion 90. The upper electrode layer 93 and the lower electrode layer 94 sandwich the piezoelectric layer 92. A cavity portion 95 penetrating the substrate 91 is provided such that the lower surface of the piezoelectric vibrating portion 90 is exposed. The cavity portion 95 is provided in the substrate 91 in order to ensure free vibration of the piezoelectric vibrating portion 90.
When an electric field is applied between the upper electrode layer 93 and the lower electrode layer 94, electrical energy is converted into mechanical energy in the piezoelectric layer 92. For example, when aluminum nitride (AlN) having its polarization axis in the thickness direction is used as the piezoelectric layer 92, the mechanical energy is converted principally into a longitudinal vibration in the thickness direction. Thus, the piezoelectric layer 92 expands and contracts in the same direction as the electric field.
As shown in FIG. 27B, the equivalent circuit diagram of the conventional piezoelectric element includes a serial resonant circuit and a parallel resonant circuit. Therefore, the conventional piezoelectric element has a resonant frequency and an anti-resonant frequency. When the thickness of the piezoelectric vibrating portion 90 is taken as t, the conventional piezoelectric element resonates at a resonant frequency fr (=v/λ) corresponding to a wavelength λ that satisfies t=λ/2, where v is the ultrasonic velocity in the material constituting the piezoelectric vibrating portion 90. The anti-resonant frequency fa, similarly to the resonant frequency, is inversely proportional to the thickness t of the piezoelectric vibrating portion 90, and is proportional to the ultrasonic velocity in the material constituting the piezoelectric vibrating portion 90. When the resonant frequency and/or the anti-resonant frequency is to be set within a frequency band of several 100 MHz to several GHz, the thickness of the piezoelectric vibrating portion 90 corresponding to such a resonant frequency and/or anti-resonant frequency is a thickness that can be formed easily by industrial thin-film formation. Therefore, the conventional piezoelectric element is compact and is useful as a resonator having a high Q-value in the above-described frequency band.
In the piezoelectric vibrating portion 90, ideally, it is preferable that only vibrations in the thickness direction P of the piezoelectric layer 92 are present. However, the piezoelectric vibrating portion 90 is supported at its periphery by the substrate 91, and therefore the piezoelectric vibrating portion is constrained by the supporting portion in the substrate 91. Therefore, spurious responses tend to occur.
Furthermore, in the conventional piezoelectric element, vibrations in the lateral direction Q are also excited, so that a plurality of laterally propagating acoustic wave modes are present. These laterally propagating acoustic wave modes are unwanted vibration modes. The laterally propagating acoustic wave modes propagate in parallel to the surface of the electrode and effect multiple reflections at the side wall of the piezoelectric layer 92 or the end portions of the upper electrode layer 93 and the lower electrode layer 94, which causes spurious response. Furthermore, in the case of a composite piezoelectric element in which piezoelectric elements are arranged adjacent to each other, unwanted vibration modes interfere with each other between the adjacent piezoelectric elements, which causes spurious response. The spurious responses that are caused by the laterally propagating acoustic wave modes deteriorate the frequency characteristics of the piezoelectric element.
Various techniques have been proposed in order to solve the problem that the frequency characteristics of the piezoelectric element is deteriorated by the spurious responses. FIG. 28A is a top view of a conventional piezoelectric element disclosed in International Publication No. 98/52280. FIG. 28B is a cross-sectional view taken along the line B-B of the conventional piezoelectric element shown in FIG. 28A.
As shown in FIG. 28A and 28B, an insulating film 73 is formed on a substrate 81 having a cavity portion 85. A piezoelectric vibrating portion 80 is provided above the cavity portion 85, straddling the cavity portion 85. The opposite ends of the piezoelectric vibrating portion 80 are supported by the substrate 81. A lower electrode layer 84, a piezoelectric layer 82 and an upper electrode layer 83 are laminated to form the piezoelectric vibrating portion 80. The lower electrode layer 84 is connected to a wiring electrode 86. The end portion of the wiring electrode 86 constitutes a terminal electrode 87. The upper electrode layer 83 is connected to a terminal electrode 71 via a connection portion 88, a wiring electrode 89 and a connection portion 70. A mask 72 is a mask for forming the cavity portion 85. According to the conventional piezoelectric element shown in FIGS. 28A and 28B, the piezoelectric vibrating portion 80 is supported above the cavity portion 85 by the substrate 81 with two supporting portions 74 and 75. Therefore, the constraint from the substrate 81 on the piezoelectric vibrating portion 80 becomes small. Thus, spurious responses are suppressed to some extent. However, the piezoelectric vibrating portion 80 is supported at symmetric positions with respect to the substrate 81, so that in particular, the laterally propagating acoustic wave modes cannot sufficiently be suppressed.
FIG. 29A is a top view of another conventional piezoelectric element disclosed in Japanese Laid-Open Patent Publication No. 9-130199. FIG. 29B is a cross-sectional view taken along the line B-B of the conventional piezoelectric element shown in FIG. 29A.
As shown in FIGS. 29A and 29B, a piezoelectric vibrating portion 60 is provided above a substrate 61 having a cavity portion 65, straddling the cavity portion 65 via a support member 53. A lower electrode layer 64, a piezoelectric layer 62 and an upper electrode layer 63 are laminated to form the piezoelectric vibrating portion 60. The lower electrode layer 64 is connected to a terminal electrode 67 via a wiring electrode 66. The upper electrode layer 63 is connected to a terminal electrode 51 via a wiring electrode 69. An etching hole 54 is provided for forming the cavity portion 65 in the substrate 61. A protective film 55 is provided to protect the lower electrode layer 64 from the etchant during etching. According to the conventional piezoelectric element shown in FIGS. 29A and 29B, the piezoelectric vibrating portion 60 is supported by the substrate 61 with two supporting portions 56 and 57. Therefore, the constraint from the substrate 61 on the piezoelectric vibrating portion 60 becomes small. Thus, spurious responses are suppressed to some extent. However, similarly to the above example, the piezoelectric vibrating portion 60 is supported at symmetric positions with respect to the substrate 61, so that in particular, the laterally propagating acoustic wave modes cannot sufficiently be suppressed.
Furthermore, the conventional piezoelectric element shown in FIGS. 28A and 28B has the following problem. The piezoelectric vibrating portion 80 is supported by the substrate 81 with two supporting portions 74 and 75. The supporting portions 74 and 75 are line-symmetric with respect to a line segment L. Therefore, the piezoelectric vibrating portion 80 is easily twisted or easily vibrates in the width direction. Therefore, spurious responses (e.g., unwanted vibration including twist components or unwanted lateral vibration) caused by the symmetric property of the supporting portions 74 and 75 may occur, in addition to the longitudinal vibration in the thickness direction, which is the main resonance. Thus, in the conventional piezoelectric element shown in FIGS. 28A and 28B, spurious responses cannot be suppressed effectively.
Furthermore, the conventional piezoelectric element shown in FIGS. 29A and 29B has the following problem. The piezoelectric vibrating portion 60 is supported by the substrate 61 with two supporting portions 56 and 57. The supporting portions 56 and 57 are point-symmetric with respect to a point O. Therefore, the piezoelectric vibrating portion 60 is easily twisted or easily vibrates in the width direction. Therefore, in this case as well, spurious responses (e.g., unwanted vibration including twist components or unwanted lateral vibration) caused by the symmetric property of the supporting portions 56 and 57 may occur, in addition to the longitudinal vibration in the thickness direction, which is the main resonance. Furthermore, the vibration mode that propagates in an oblique direction from the end portion of the upper electrode layer 63 or the lower electrode layer 64 is confined in the piezoelectric vibrating portion 60. The confined vibration mode causes spurious responses. Thus, in the conventional piezoelectric element shown in FIGS. 29A and 29B, spurious responses cannot be suppressed effectively.